H.L.A. Hart, answered from the texts and cited to the page.
The existence of law is one thing; its merit or demerit another. No conceptual connection holds between legal validity and moral merit: a rule may be law because it satisfies the criteria of validity laid down by its system's rule of recognition, and it may at the same time be a wicked law that ought to be disobeyed. This is the separability thesis, the central commitment of the positivist tradition I inherit from Bentham and Austin.
The thesis is not a denial that law and morality overlap in content; they overlap extensively, and any legal system that is to function at all must include rules answering to what I called the minimum content of natural law — prohibitions on violence, dishonesty, and the free taking of valued things, without which a society of beings like us could not survive.1
But the overlap is contingent. What makes a rule a legal rule is its place in the system of social rules of a particular community, not its conformity to any antecedent moral standard. And the reason for insisting on this is itself moral. The positivist thesis keeps the citizen's critical faculty free against the moral pretensions of any state.
If a rule's legal validity were settled by its moral correctness, the worst legal systems would either cease to count as legal at all (the natural lawyer's verdict, which loses the descriptive grip on what we are studying) or would be retrospectively whitewashed as morally acceptable.2 Better to say plainly: this is law, and this law is wicked, and one ought not to obey it.345
Two things have conspired to make discussion of them interminable or seemingly so. The first is that the issue has been clouded by use of grand but vague words like "Positivism" and "Natural Law." Banners have been waved and parties formed in a loud but often confused debate. Secondly, amid the shouting, too little has been said about the criteria for judging the adequacy of a definition of law.Law, Liberty, and Morality, pp. 10–11
This impartiality is what the procedural standards known to English and American lawyers as principles of 'Natural Justice' are designed to secure. Hence, though the most odious laws may be justly applied, we have, in the bare notion of applying a general rule of law, the germ at least of justice.The Concept of Law, p. 207
Take only one example: Bentham on slavery. He says the question at issue is not whether those who ar e he l d 5 2 GENERAL THEORY as slaves can reason, but simply wh ȩ ther they suffer. l l Does this not compare well with the discussion of the question in terms of whether or not there are some men whom Nature has fitted only to be the living instruments of others?Positivism and the Separation of Law and Morals, p. 58
They are ranked as supremely important, so that their pursuit is felt as duty to which other interests or desires are to be sacrificed; though conversions are possible the notion that such ideals could be adopted, changed, or eliminated by a deliberate choice is chimerical; and, finally, deviations from such ideals are 'punished' by the same conscience, guilt, and remorse as that to which social morality makes its…The Concept of Law, pp. 185–186
XII, 2d para. n. ). 'Here we touch upon the most difficult of questions. If the law is not what it ought to be; if it openly combats the principle of utility; ought we to obey it? Ought we '.. · · iolate it? Ought we to remain neuter between the law which commands an evil, and morality which forbids it? See also Bentham, A Fragment on Government, in I Works 22 1, 287- 8 (c. IV, 20th-25th paras.).Positivism and the Separation of Law and Morals, pp. 60–61
The Hart-Fuller debate (Harvard Law Review 1958) is the canonical mid-twentieth-century engagement of legal positivism with its natural-law critics. Fuller's "inner morality of law" — the eight principles of legality — was advanced as a refutation of the separability thesis; I held the principles to be efficacy conditions, not moral conditions. Finnis's Natural Law and Natural Rights (1980) is the most sophisticated natural-law restatement of the century. Raz's exclusive positivism and Coleman's inclusive positivism continue the intra-positivist dispute; the Dworkinian post-positivist critique remains the central external challenge.